It is known that the transmission power of an ac voltage network is reduced during reactive load and during uneven load of the three phases. Usually, the load is reactive and symmetrical. In this context, it is known to arrange a compensator comprising capacitors connectable to the network. It is also known to connect to the network a static compensator, usually designated SVC (Static Var Compensator) for balancing unsymmetries caused by unbalanced loads. Typical apparatus that may cause an unbalance in an ac network are, for example, arc furnaces. Another such load that may cause an unbalance is power supply of railways. Railway loads are often connected between two phase in the power network. To distribute the power output evenly between all the three phases of the power network, it is known to connect a static compensator, also called a load balancer. The compensator transfers power between the phases in such a way that the railway load together with the compensator is perceived by the power network as a symmetrical three-phase load.
A compensator is traditionally based on thyristor-controlled reactors, thyristor-controlled capacitors and fixed capacitor banks/filters. This technique is well established. A compensator may also be based on a voltage-source converter (VSC). Such a VSC comprises semiconductors with turn-off means and has a large field of operation. Use of this type of compensator, VSC, is also known.
Various devices for application within FACTS are known from, for example, U.S. Pat. No. 6,963,187.